This invention relates to a process for producing scorodite. Especially, it relates to a process for producing scorodite from electrolytically precipitated copper produced during a step for smelting copper. Further, the present invention also relates to a process for recycling the post-scorodite-synthesis solution.
Copper ore contains a variety of impurities such as arsenic (As). Arsenic (As) is separated by volatilization at high temperatures during a dry process for copper refining, but partly remains in crude copper before an electrolytic refining step.
As contained in the crude copper (copper anode) is partly eluted in an electrolytic solution, while the uneluted As is contained in the anode slime that is precipitated on the bottom of the electrolytic bath. Since the copper volume deposited on the cathode is generally larger than that eluted from the anode, the copper content in the electrolytic solution gradually increases. Part of the electrolytic solution is thus transferred to another electrolytic bath to control the quality of the electrolytic solution. The transferred electrolytic solution is subjected to decoppering electrolysis. Impurities such as Cu and As are deposited on the cathode and precipitated on the bottom of the electrolytic bath, which can be recovered. The precipitate on the bottom of the electrolytic bath and the deposition on the cathode are collectively referred to as electrolytically precipitated copper in the art.
Typically, electrolytically precipitated copper is repeatedly fed back to the smelting step. For this purpose, the impurities such as As are preferably separated from the electrolytically precipitated copper. In addition, As may be used as valuables. Thus, there is a need for a technique for individually separating and recovering and fixing As of high quality level from electrolytically precipitated copper. It is known that producing a crystalline of scorodite (FeAsO4.2H2O) which is a compound of iron and arsenic is useful for fixing arsenic. A crystalline scorodite is chemically stable, and suitable for long-term storage. On the other side, amorphous scorodite lacks the stability and therefore it is not suitable for long-term storage.
Heretofore, it is a process, in which trivalent iron is added to a solution containing pentavalent arsenic and a heat-treatment is performed at the temperature equal to or over 80° C. under acidic condition so that a crystalline scorodite is produced, that has been generally employed as a process for producing a crystalline scorodite from a solution containing arsenic. This technique is described in detail, for example, in Japanese Patent Publication No. 3756687 “Process for eliminating arsenic from a solution containing arsenic and fixing it”, Japanese Patent Application Public Disclosure No. 2005-161123 “Process for eliminating arsenic from a smoke ash”, and Japanese Patent Application Public Disclosure No. 11-277075 “Process for eliminating arsenic present in a solution of iron sulfate and fixing it”.
In these documents, disclosures about Fe/As during synthesis of scorodite, iron concentration of a post-scorodite-synthesis solution, and recovery of copper from the post-scorodite-synthesis solution are given as follows.
It is defined, in Japanese Patent Publication No. 3756687, that Fe/As=1.5-2.0, which is required for improving the crystallinity of arsenic compounds produced, and repressing the eluation of arsenic. It is described that when scorodite is synthesized using the molar ratio outside the above-described range, the crystallinity remarkably decreases and arsenic is liable to elute. In working example in said patent publication, “3 L of a solution containing iron in which Fe is contained at 40 g/L was added to 6 L of the arsenic containing solution so that Fe/As (in molar ratio) may become 1.8, and then the solution was encapsulated into an autoclave, and the temperature was increased to 165° C.” is described. Further, with regard to recovery of copper, “zinc powder is added to the post-dearsenic solution obtained and decoppering treatment is applied to the solution, and after recovery of copper, said solution is added to the leached solution together with a post-deiron solution in the above-described process for treatment of the residue of zinc leaching, and then the same treatments are repeated” is disclosed. There is no description about the concentration of iron in the post-scorodite-synthesis solution.
While it is defined, in Japanese Patent Application Public Disclosure No. 2005-161123, that Fe/As=1.0-1.5, the reason why the ratio is so defined is not disclosed. In working example thereof, while “an aqueous solution of ferric sulfate (concentration of iron ion (Fe3+) is 80 g/L) at pH 1.0 was added to a leached solution at pH 1.0 obtained by leaching out arsenic from the smoke ash using a solution of sulfuric acid and performing a filtration, so that molar ratio of iron to arsenic may become 1-1.5, and then the mixture was heated to 95° C. under the condition that the concentration of arsenic is equal to or more than 10 g/L to crystallize an amorphous iron arsenate” and “an aqueous solution of ferric sulfate was added so that the molar ratio of iron to arsenic may become 1.3-1.5, and then the mixture was heated to 95° C. under the condition that the concentration of arsenic is 15 g/L to crystallize an amorphous iron arsenate” are described, Fe/As ratio and the concentration of iron in the post-scorodite-synthesis solution are not disclosed clearly. Further, with regard to recovery of copper, “a post-dearsenic solution obtained by filtration of the crystalline of iron arsenate (FeAsO4.2H2O) is separated into a copper solution and a crude zinc solution, by a solvent extraction, and then the copper solution is electrolytically processed for recovery of copper. The crude zinc solution is subjected to a primary neutralizing treatment, and then, separated, by solid-liquid-separation, into a liquid residue and an iron and arsenic precipitate containing amorphous arsenic, said residue being subjected to a given treatment. Subsequently, said iron and arsenic precipitate containing amorphous arsenic is added again to the leached solution, and the crystallization treatment of step S14 is performed” is disclosed.
In Japanese Patent Application Public Disclosure No. 11-277075, “For obtaining a sufficient precipitation ratio of arsenic, mixing is preferably performed so that 1<Fe/As<3 or Fe/As>10 may be realized, since the speed of forming the precipitate of the compounds of iron and arsenic is slow in the range 3<Fe</As<10” is described. In the working examples, Fe/As was 4 (Example 1), 1.3 (Example 2), and 4.5 (Example 3). While the concentration of iron in the post-scorodite-synthesis solution is not disclosed, it is described that copper is recovered or reused without recovery as a source of copper in a dearsenic process.